Synthesis of riboflavin and intermediates therefor



Patented Sept. 3, 1946 SYNTHESIS OF RIBOFLAVIN AND INTERMEDIATESTHEREFOR Jonas Kamlet, New York, N. Y., assignor to Miles Laboratories,Inc., Eikhart, Ind, a corporation oi Indiana No Drawing. ApplicationJune 28, 1943,

' Serial No. 492,622

13 Claims. 1

The present invention relates to a new synthesis or riboflavin. Moreparticularly it relates to a new process whereby 6,7-dime'thyl-9-(d-1'-ribityl) isoalloxazine, which is identical with the naturally occurringriboflavin, or vitamin B2, may be synthesized. It has for an object toprovide a simplified procedure for the synthesis of this vitamin,whereby some steps in the present synthesis may be eliminated. It hasfor a further object to provide a synthesis of riboflavin which does notrequire the use of the expensive and difficultly-obtainable pentosesugar, d-ribose.

In 1933, Gyorgy, Kuhn and Wagner-Jauregg, isolated riboflavin andrecognized its identity with vitamin B2 (Naturwissenschaften, 21, 560-1of 1933). Shortly afterward, the structure of this vitamin wasestablished to be 6,7-dimethyl-9-(d- 1'-ribityl) isoalloxazine (CompoundA) (Compound A) $HZ-CHOH-CHOH.CHOE'CHZOH and it wa synthesized by Karrer(Helv. chim. Acta, 18, 522-35 of 1935), and Kuhn (Berichte, 68, 176574of 1935).

The basis of both the Kuhn and the Karrer processes is the condensationof 3,4-dimethyl-6- amino-phenyl-d-ribamine with alloxan to yieldriboflavin. Karrer carries this condensation out in acid solutions (U.S. Patents 2,155,555 and 2,237,074), whereas Kuhn materially increasesthe yield of that condensation by efiecting it in glacial acetic acidsolution with the use of boric acid as a catalyst (U. S. Patent2,238,874; Berichte, 68, 1282 of 1935). The 3,4-dimethyl-6-aminophenyl-d-ribamine required for this condensation with alloxan maybe prepared by a number of methods as follows:

(a) 4,5-Dinitro-o-Xylene is condensed with dribamine and reducedcatalytically in aqueous a1- coholic solution (Kuhn and Weygand,Berichte, 68, 1001 of 1935). The yields by this method are too low to bepractical.

(b) 3,4-dimethyl, G-nitroaniline is condensed with d-ribose and reduced(Kuhn et al., Berichte, 68, 1765 of 1935; 70, 773 of 1937) 3,4 dimethyl,6 carbethoxyaminoaniline (Karrer et al., Helv. chim. Acta, 18, 69 of1935; 18, 426 of 1935), or 3,4-dimethyl, G-acetylaminoaniline (Karrer etal., Berichte, 68, 216 of 1935) is condensed with d-ribose, reduced andthen saponified to the free amine, 3,4-dimethyl,G-aminophenyl-d-ribarnine.

(d) SA-dimethylaniline is condensed with dribose and the resultantriboside is catalytically reduced to 3,4-dimethy1phenyl-d-ribamine. Thiscompound is then coupled with an aryl diazonium salt to form the3,4-dimethyl, fi-arylazophenyl-dribamine and the latter compound isreduced to the 3,4-dimethyl, G-aminophenyl d ribamine. (Karrer et al.,Helv. chim. Acta, 18, 1435 of 1935.) This method gives the best yieldsof all of those described.

It will be noted that all of the methods described for the preparationof 3,4-dimethyl, 6-aminophenyl-d-ribamine require the use of d-ribose,or

a derivative thereof. d-Ribose may be derived from natural sources, orprepared synthetically, but, under any circumstances, it is verydiflicult and expensive'to obtain. The major cost in the synthesis ofriboflavin may be attributed to the d-ribose required. In order to avoidthe use of this expensive pentose, Weygand (Berichte, 73, 1264 of 1940)made novel use of a valuable organic reaction, the Amadorirearrangement. By this method, 3,4-dimethylaniline is condensed withd-arabinose. The resultant d-arabinoside, in the presence of a smallamount of acid at an elevated temperature rearranges to form the 3,4-dimethylphenyl-d isoarabinosamine, which is then reduced in alkalinesolution to the 3,4-dimethylphenyl-d-ribamine. This compound can then becoupled with a diazonium salt and reduced, as described in paragraph(11) above. Although the overall yield'of 3,4-dimethyl,(i-aminophenyl-d-ribamine is low (9-10%), the raw materials required forthis synthesis are inexpensive. d-Arabinose can be prepared in goodyield by submitting the low-cost and readilyavailablecalcium-d-gluconate to a Rufi" degradation, i. e., oxidationwith hydrogen peroxide in the presence 01 ferric acetate (Hockett andHudson, Journ. Amer. 'Chem. Soc, 56, 1632 of 1934). t

It is an object of the present invention to improve and simplify theprocedure whereby the Amadori rearrangement may be used for thepreparation of 3,4-dimethyl, 6-aminophenyl-dribamine, as well as toprovide a novel synthesis of riboflavin that avoids the direct use of3,4- dimethyl, G-aminophenyl-d-ribamine, but employs rather a moreeasily obtained precursor thereof, the 3,4-dimethyl,G-aminophenyI-disoarabinosarnine.

According to Kuhn and Weygand (Berichte, 70, 769 of 1937) the Amadorirearrangement involves the conversion of arylamine-N-aldosides into thecorresponding aryl-N-isoketosamines, in the presence of a small amountof acid at elevated temperatures. Thus, when the aldose is darabinose,the rearrangement probably goes through the following stages, catalyzedby the acid present:

The Schifi base (I) first formed is in equilibrium with the furanoseamino-aldoside (II). The oxygen-bridge of the latter readily breaks toform the -ol form (III) of the isoarabinosamine, which tautomerizes totheketo form (IV). When the original amine R.NII2 is3,4-dimethylaniline, the final isoketosamine is the 3,4-dimethylpheny1-d-isoarabinosamine. To eiTect this rearrangement, 1 mole of the aldose,1.1 to 1.4 moles of the aryl amine and 0.002 to 0.02 mole of acid areheated in the presence of 2 to 4 moles of water at 70-90 C. for a fewminutes. (Weygand; Berichte, 73, 12591276 of 1940.) The final product isa mixture of the arylamino-N- alcloside and the aryl-N-isoketosamine.

In the specification and claims of this invention, the term isoaribity(see Compound M) (q. v. Weygand) refers either to the Z-keto-daribitylor to the Z-keto-d-ribityl group, or to both. The only difierencebetween d-arabinose and d-ribose resides in the optical configuration ofthe asymmetric carbon in position 2. When this center of asymmetry iseliminated (e. g., by conversion to a keto group) both cl-arabinose andd-ribose give the same Z-ketopentose. Thus, 2- keto-d-ribityl is thesame group as Z-keto-daribityl, and is here referred to asd-isoaribityl.

The present invention is based on the following sequence of reactions:

(at) 3,4-dimethy1aniline (Compound B) and darabinose (Compound C) aresubmitted to an Amadori rearrangement. The resultant mixture of3.4-dimethylphenyl-d-isoarabinosamine (Compound D) and3,4-dimethylaniline-d-arabinoside (Compound D) is then distilled withsteam. The isoarabinosamine (Compound D) is not affected by thistreatment, but the arbinoside (Compound D) readily hydrolyzes to givback the original BA-dimethylaniline and d-arabinose. TheBA-dimethylaniline distills over with the steam and the major portion ofthe amine originally started with may thus be recovered for reuse.

The residue after the steam distillation comprises an oily layer ofimpure 3,4-dimethylphenylylphenyl-d-isoarabinosamine (Compound D) is nowdissolved in a suitable organic solvent (e. g., ethanol, glacial aceticacid), treated with a small amount of decolorized carbon and filtered.

(b) The solution of 3,4-dimethylphenyl-d-isoarabinosamine (Compound D)is now coupled with a diazotized arylamine (Compound E), of which thearyl nucleus is eventually removed,

' K while a N-atom 0f the diazo linkage is retained.

Although any arylamine may conveniently be used, I prefer to use anarylamine containing a water-solubilizing group on the ring, as inpaminobenzoic acid, and sulfanilic acid, and many others which areavailable. The rationale for using an arylamine bearing aWater-solubilizing group on the ring will become evident as this processis elaborated.

The reaction involved in this step is the following:

(Compound D) (Compound F) CH3 NHCH2COCHOHCHOHCH20H correspondingsecondary alcohol (Compound H).

These agents (Compound G) include sodium amalgam in the presence of astream of CO2 (to neutralize the NaOH evolved), hydrogen in the presenceof Fancy nickel, and hydrogen in the presence of a reduced platinumcatalyst. As Weygand has postulated, reduction of the d-isoarabinosamineside chain in alkaline solution leads only to the desired d-ribamin(Compound H) (Berichte, 73, 1262 of 1940), thus:

(Compound F) OH3- -NH.OH .CO.OH0H.CHOH.CH2OH CH3 -N=NR reduction(Compound G) (Compound H) CH -NH.CH2CHOH.CHOH.CHOH.CH2OH CH3 NHQ R.NH2

The reaction mixture, after reduction, is evaporated in vacuo todryness. The residue is dissolved in a minimum amount of hot absoluteethanol, and filtered from insoluble material. If the original amineused for obtaining the diazotized arylamine contained a olubilizinggroup, i. e., if it was p-aminobenzoic acid, for example, it exists inthe dried residue as the alkali-metal salt which is soluble in water butinsoluble in the absolute alcohol. Thus, a convenient method is providedfor the separation of the 3,4-dimethyl, fi-aminophenyl-d-ribamine(Compound H) and the original coupling amine that was regenerated by thereduction.

The 3,4-dimethyl, G-aminophenyl-d-ribamine (Compound H) thus obtainedmay now be converted to riboflavin by condensation with alloxan, asdescribed by Karrar and Kuhn (q. v. supra) (Compound L).

2. Reducing agent (Compound J) that will reduce only the diazo linkage.These agents include sodium hydrosulfite (Na2SzO4) in alkaline solution,and nascent hydrogen in acid solution as produced by zinc dust in acidsolution. The 3,4 dimethyl, 6 arylazophenyl d isoarabinosamine (CompoundF) will thus be reduced to a new compound, 3,4-dimethyl,G-aminophenyl-d-isoarabinosamine. (Compound K) (Compound F) CHNH.CH:CO.CHOH.CHOH.CH2OH a reduction CH; N=NR (Compound J) (Compound K)CHz- 'NH.CH2.00.0HOH.CHOH-CHIOH CHr- NH: 3.1m,

of 6,7-dimethyl, 9-(d-1'-ribityl) isoalloxazine (Compound P) (CompoundM) 0112.0 O.CHOH.CHOH.OH:OH

which rapidly reverts to riboflavin (Compound A) onshaking with air(Compound Q). The catalyst is, filtered oif and the riboflavinisrecovered from the alcoholic filtrate by evaporating 'to dryness invacuo. It is obvious, of course, that in this condensation, the use ofboric acid as a catalyst is optional, and the use of alloxan, asmentioned in this specification and claims likewise refers to itsfunctional equivalents, such as dialuric acid, isodialuric acid andalloxantine.

The following examples are given in order to define and illustrate thisinvention but in no way to limit it to reagents, proportions orconditions described therein. Numerous modificanow condensed (CompoundL) in glacial acetic acid with alloxan in the presence of boric acid asa catalyst, to yield a new compound 6,7-dimethyl, 9-(d-l-isoaribitylisoalloxazine (Compound M) (CompoundK) CH3 NH.CH'2.CO.CHOH.CHOH.CH2OH CHNH:

(Compound L) (Compound M) CH2.CO.CHOH.CHOH.CH2OH I CH? C CO 00 co I l 30l TH CH@ N CO The acetic acid (Compound M) is distilled oif in vacuo,the yellow residue of impure, 6,7-dimethy1, 9-(d-1'-iscaribityl)isoalloxazine is dissolved in a 0.2N sodium hydroxide solution inethanol and the solution is hydrogenerated in the presence of a reducedplatinum catalyst (the agent being herein designated Compound N forconvenience). The 6,7-dimethyl, 9-(d1'-isoaribityl) isoalloxazine(Compound M) is reduced to the leuco form tions will occur to any personskilled in the art.

EXAMPLE I (Compound F) and (a, species of 3,4-dimethyl,

6-arylaeophenyl-d-isoarabinosamine) A mixture of 20.0 grams ofd-arabinose, 16.0 grams of 3,4-dimethylaniline (Compound B), 1.0 gram ofbenzoic acid and 6.0 cc. of water is heated on the boiling water bathfor six minutes. A homogeneous melt is soon obtained which is a mixtureof, 3,4-dimethylphenyl-d-isoarabinosamine (Compound D) and3,4-dimethylanilined-arabinoside (Compound D). 100 cc. of hot alcohol isnow added, and the solution is submitted to steam distillation until thedistillate is substantially iree of 3,4-dimethylaniline.

The residue in. the distilling flask soon separates into two phases. Theaqueous phase (containing the major portion of the d-arabinoseoriginally used) is decanted and preserved. The dark brown oily residueis dissolved in 500 cc, of 95% ethanol, 2.0 grams of decolorizing carbonis added, the mixture is heated on the water-bath under reflux for 30minutes and then filtered.

Simultaneously, a diazo solution is prepared by dissolving 2.75 grams ofp-aminobenzoic acid and 4.0 cc. of concentrated hydrochloric acid in 50cc. of ice water, chilling the solution to 5-l0 C., then adding dropwisewith constant stirring,

14.0 cc. of 10% sodium nitrite solution.

The filtered alcoholic solution of 3,4-dimethylphenyl-d-isoarabinosamine(Compound D) is cooled, with constant stirring, to 10-15 C., and

the freshly prepared diazo solution (Compound E) 'is added slowly.Stirring is continued for an hour, and the solution is then madealkaline by the cautious addition of concentrated alcoholic NaOHsolution. There is thus obtained a dark reddish-brownsolution-suspension of the sodium salt of 3,4-dimethyl,G-(p-carboxyphenylazo) phenyl-d-isoarabinosamine (Compound F).

The free acid may be obtained by diluting the alcoholic solution withthree volumes of water and acidifying with acetic acid. A gummy reddishbrown material separates out which rapidly solidifies and may becomminuted to amorphous brown particles, In. pt. 838 l (decomp.). For3,4-dimethyl, 6-(p-carboxyphenylazo) phenyl-disoarabinosamine (CompoundF): N (calculated) =10.5%; N (-found)=10.7%. The yield is 5.1 grams.

Step 2.Pre1vamtion of 3,4-dimethyl, 6-aminophenyZ-d-ribamine (CompoundH) The alcoholic solution-suspensionof the sodium salt of 3,4-dimethyl,6-(p carboxyphenylazo) phenyl-d-isoarabinosamine (Compound F), (preparedin step 1) is made 0.2N with respect to free sodium hydroxide, and acatalyst prepared from 3.0 grams of chlorplatinic acid according to themethod of Adams in J our. Amer. Chem. Soc. 44, 1937 (1922); 415, 2171(1923) suspended in alcohol, is added. Hydrogen gas (Compound G) is nowpassed through the vigorously agitated solution at 25-80 C. until thereduction is complete and no more hydrogen is absorbed. The catalyst, aswell as other insoluble material, is now filtered off and the alcoholicfiltrate is evaporated to dryness in vacuo. The dry residue is taken upin 100 cc. of hot absolute ethanol, filtered from insoluble material andevaporated to dryness. There are thus obtained 4.0 grams of light orangecolored crystals, in. pt. 123124 C. For 3,4-dimethyl,S-aminophenyl-d-ribamine (Compound H): (calculated)=l0.4%; N (found)=10.4%.

This 3,4-dimethyl, G-aminophenyl-d-ribamine (Compound I-l) may noW beconverted to ribofiavin by the methods of Karrer or Kuhn.

EXAMPLE II Step 1.Preparation of 3,4-dzmethyl, d-(p-c'arbomyphenylazo):henyZ-d-isoambmosamine This step is eifected as described in Example I.By acidifying the steam-distillate with hydrochloric acid and distillingon" the solvent, 10 grams of 3,4-dimethylaniline (Compound B) may berecovered (as the hydrochloride), and returned to the process.Similarly, from the aque ous phase of the distillation residue 12 gramsof d-arabinose (Compound C) may be precipitated as the2,4-dinitrophenylhydrazone. By adding this aqueous phase to the crudesolution of darabinose obtained by following the procedure of Hockettand Hudson, immediately prior to the first decolorization with carbon(Jour. Amer.

Chem. Soc. 56, pg. 1633, column 1, line '7, of July, 1934), there may berecovered 10 grams of darabinose in addition .to the normal yield of 55to 65 grams. Thus, the final yield of 5.1 grams of 3,4-dimethyl,6-(carboxyphenylazo) phenyld-isoarabinosamine (Compound F) is obtainedfrom an overall consumption of 6.0 grams of 3,4- dimethylaniline and10.0 grams of d-arabinose.

Step 2.--Preparation of 3,4-dzmethyl, zi-amz'nophenyl-d-isoarabinosamine(Compound K) The alcoholic solution-suspension of the sodium salt of3,4-dimethyl, G-(carboxyphenylazo) phenyl-d-isoarabinosamine (CompoundF) (prepared in step 1) is heated under reflux on the boiling water-bathand a saturated aqueous solution of sodium hydrosulfite (Compound J) isadded slowly until the dark reddish-brown solution has been dischargedto a dark orange color, and there is an excess of free reducing agent inStep 3.-Preparation of riboflavin 1.05 grams of 3,4-dimethyl,G-aminophenyl-disoarabinosamine (Compound K) is dissolved in 60 cc. ofglacial acetic acid and added to a solution of 0.95 gram of alloxantetrahydrate (Compound L) and 1.80 grams of boric acid in 60 cc. ofboiling glacial acetic acid. The mixture is gently refluxed in the darkfor 15 minutes and the acetic acid is then distilled ed in vacuo.

The crude residue of GII-dimethyl, 9-(d-lisoaribityl) isoalloxazine(Compound M) thus obtained is dissolved in 200 cc. of 0.2N NaOH inethanol and filtered from insoluble material. The Adams catalystprepared from 1.5 grams of chlorplatinic acid, suspended in alcohol, isadded and hydrogen gas is passed through the vigorously agitatedsolution at 2530 C. until the reduction is complete and no more hydrogenis absorbed. The catalyst is filtered ofi and the filtrate, comprisingan alcoholic solution of leuco- 6,7-dimethyl-9-(d-1ribityl)isoalloxazine (Compound P) is neutralized with acetic acid. It is thenaerated for an hour to oxidize the leuco base (Compound P) b the oxygen(Compound Q) of the air and evaporated to dryness in vacuo. From thecrude residue, riboflavin (Compound A) may be recovered oncrystallization.

It is to be observed that in Example 2 the reducing agent (Compound J)is employed which reduces only the N=N-- group but not the -CO-- groupof Compound F, thus producing Compound K, which couples with the alloxanor equivalent Compound L to form Compound M; While in Example 1, areducing agent (Compound G) is used which reduces both of said groups-N=N and CO forming Compound H, which then coupled with a Compound Lprovides Compound M which is similar to Compound M in all respectsexcept that the -CO-- group of Compound M is a CHOH- group in CompoundM. The reducing agent Compound N reacts with either Compound M orCompound M to give the leuco product (Compound P), which is oxidizableby air to riboflavin (Compound 1A) Having described my invention, what Iclaim and desire to protect by Letters Patent is:

1. In the process of making riboflavin the steps of reacting d-arabinosewith 3,4-dimethylaniline and thereby forming the compound3,4-dimethylphenyl-d-isoarabinosamine, reacting the latter compound witha salt of a diazotized arylamine and thereby forming the compound3,4-climethyl, 6-arylazophenyl-d-isoarabinosamine, and reducing thelatter compound by the action of a re ducing agent selected from thegroup consisting of alkali metal hydrosulfites in neutral solution andin alkaline solution and nascent hydrogen in acid solution therebyforming 3,4-dimethyl, 6- aminophenyl-d-isoarabinosamine of the formula:

CH3 -NH.CH2.CO.CHOH.CHOH.CH:OH

2. In the process of making riboflavin the steps of reacting,d-ar'abinosewith 3,4-dimethylaniline i metal hydrosulfltes in neutralsolution and in alkaline solution and nascent hydrogen in acid solutionthereby forming the compound ISA-die methyl,6-aminophenyl-d-isoarabinosamine, and condensing the latter compoundwith alloxan thereby forming 6,7-dimethyl, 9- (d-l-isoaribityl)isoalloxazine of the formula:

onlooononononomon acid solution to form 3,4-dimethyl,6-aminophenyl-d-isoarabinosamine, condensing the latter with, alloxan inacid solution to form 6,7-dimethyl, 9-(d-1-isoaribityl) isoalloxa zine,reducing the latter in alkaline solution to theleuco form of riboflavinby the action of hydrogen acting in the presence of ahydrogenationcatalyst, and oxidizing said leuco form to riboflavin.

4. In a process for the synthesis of riboflavin, the step wh chcomprises reducing 3,4-dimethyl, 6-arylazophenyl-d-isoarablnosamine to3,4-dimethyl, U-aminopnenyl-ci-rioamine by the action of a reducingagentselected from the group consisting or (1) 500111111 amalgam, and(2) hydrogen acting in the presence of a hydrogenation catalyst.

5. In a process for the synthesis of riboflavin, the step whichcomprises reducing 3,4-dlmethyl, fi-ip-carboxyphenylazo)phenyl-cl-isoarabinos2imine to 3,4-almethyl, 6-aminophenyl-dribamine bythe action of a reducing agent selected from the group consisting of (1)sodium amalgam, and k mine to 3,4-dimethyl,G-aminophenyl-d-isoarabinosamine by the action of a reducing agentselected from the group consisting of alkali metal hydrosulfites inneutral solution and'in alkaline solution and nascent hydrogen inacidsolution.

8. In a process for the synthesis of riboflavin, the step whichcomprises condensing 3A-dimethy1,

10$ 6-aminophenyl-d-isoarabinosamine with alloxan in acid solution. l v

.,9.;In .a process "for the synthesis of riboflavin, the; step whichcomprises condensing 3,4-dimethyl, 6 eaminophenyl-d-isoarabinosaminewith alloxan in acetic acid solution.

1 0; lira, process for the synthesis of. riboflavin, the step whichcomprises reducing 6,7-dimethyl- 9-(d 1"-is0aribityl) isoalloxazine bythe action of hydrogen acting in the presence of a hydrogenationcatalyst, anddehydrogenating the resulting leuco-BJT -dimethyl,9-(d1-,ribityl) 'isoalloxazine thereby forming riboflavin.

11. The process of producing riboflavin which comprises reacting3,4-dimethylaniline and darabinose by an Amadori rearrangement whileforming thereby 3,4-dimethyl-phenyl-disoara=- binosamine, coupling thelatter with a diazonium salt of a diazotized aromatic amine therebyforming 3,4-dimethyl, 6-arylazophenyl-d-isoarabinosamine, reducing thelatter by the action of a reducing agent selected from the groupconsisting of (1) sodium amalgam, and (2) hydrogen acting in thepresence of a hydrogenation catalyst thereby forming 3,4-dimethyl,6-aminopheny1- d-ribamine, and condensing the latter with an agentselected from the'group consisting of a1- loxan, dialuric acid,isodia'luric acid and alloxantine.

12. The process of producing riboflavin which comprises reacting3,4-dimethylaniline and darabinose by an Amadori rearrangement whileforming thereby 3,4-dimethylphenyl-d-isoarabinosamine, coupling thelatter With a diazonium Y salt of a diazotized aromatic amine therebyforming 3,4-dimethyl, ,6-arylazophenyl-d-isoarabinosamine, the lattercontaining a benzene nucleus having attached thereto the linkage N=N-,reducing the N=N group by the action of a reducing agent selected fromthe group consisting of (1) sodium amalgam, (2) hydrogen in the presenceof a hydrogenation catalyst, (3) alkali metal hydrosuliites in neutralsolution and in alkaline solution, and (4) nascent hydrogen in acidsolution, thereby forming a compound having the resulting group on N:

CH:- NH2 condensing the said resulting compound with a compound selectedfrom the group consisting of alloxan, dialuric acid, isodialuric acidand alloxantine, While thereby forming the essential grouping andreducing the resulting compound by the action of hydrogen and ahydrogenation catalyst thereby forming the leuco base of riboflavin fromwhich riboflavin is obtained by oxidation.

13. .The process of producing riboflavin which comprises reacting3,4-dimethylaniline and darabinose by an Amadori rearrangement whileforming thereby 3,4-dimethylphenyl-d-isoarabinosamine, coupling thelatter with a diazonium salt of a diazotized aromatic amine therebyforming 3,4-dimethyl, 6-arylazophenyl-d-isoarabinosamine, the lattercontaining a benzene nucleus having attached thereto the linkage -N=N-,

loxan, dialuric acid, isodialuric acid and alloxantine thereby formingthe essential grouping reducing the said essential grouping by theaction of hydrogen in the presence of a hydrogena- 10 tion catalyst tothe leuco base of riboflavin from which riboflavin is obtained byoxidation. 7

JONAS KAMLET.

